libstdc++: Improve doxygen documentation for std::unique_ptr

Add more detailed documentation for unique_ptr and related components.

The new alias templates for the _MakeUniq SFINAE helper make the
generated docs look better too.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/unique_ptr.h (default_delete): Add @since tag.
	(unique_ptr, unique_ptr<T[]>): Likewise. Improve @brief.
	(make_unique, make_unique_for_overwrite): Likewise. Add @tparam,
	@param, and @returns.
	(_MakeUniq): Move to __detail namespace. Add alias template
	helpers.